There has been substantial success in the production of proteins in mammalian cells and bacteria, which has been the primary focus and success of genetic engineering, while progress with the genetic engineering of plants has proven to be of substantially greater difficulty. With plants, one must usually modify the naturally occurring plant cell in a manner in which the cell can be used to generate a plant. Even in the event that this is successful, it is frequently essential that the modification be subject to regulation. That is, it will be of interest that the particular gene be regulated as to the differentiation of the cells and maturation of the plant tissue. There is also interest as to the site where the product is directed within the plant cell. Thus, there are substantially increased degrees of difficulty in genetically engineering plants.
Furthermore, plants have a larger number of chromosomes than the mammalian genome. Isolating specific genes and their regulatory regions in plants requires a major effort. Associated with this effort is the need to isolate DNA from a library, device techniques for demonstrating the presence of the gene on a particular fragment, isolating the gene from the fragment, providing that the gene is the correct gene, verifying that the product of the gene is the correct protein, and manipulating the gene so that it may be used for an intended purpose.
The path for genetic engineering of plants is a long and arduous one, further exacerbated by the need to go from cells to plants, which greatly extends the period of time before one can establish the utility of one's genetic construction. There is the further concern of the generality of the construction as to its use in different plant species. In addition, there is the necessary screening, where one wishes to localize the expresssion of the particular construction in particular cell types and the further concern that the genetically modified plant retain the genetic modification through a plurality of generations.